Nickel-base superalloys are known. Single-crystal components made from these alloys have a very good material strength at high temperatures. This allows, for example, the intake temperature of gas turbines to be increased, with the result that the efficiency of the gas turbine rises.
Nickel-base superalloys for single-crystal components, as are described in, for example, U.S. Pat. No. 4,643,782, EP 0 208 645 and U.S. Pat. No. 5,270,123, for this purpose contain alloying elements which strengthen the solid solution, for example Re, W, Mo, Co, Cr, and elements which form γ′ phases, for example Al, Ta and Ti. The level of high-melting alloying elements (W, Mo, Re) in the basic matrix (austenitic γ phase) increases continuously as the temperature of load on the alloy increases. For example, standard nickel-base superalloys for single crystals contain 6-8% W, up to 6% Re and up to 2% Mo (details in % by weight). The alloys disclosed in the abovementioned documents have a high creep strength, good LCF (low cycle fatigue) and HCF (high cycle fatigue) properties and a high resistance to oxidation.
These known alloys were developed for aircraft turbines and were therefore optimized for short-term and medium-term use, i.e. the load time was designed for up to 20,000 hours. By contrast, industrial gas turbine components have to be designed for a load time of up to 75,000 hours.
By way of example, after a load time of 300 hours, the alloy CMSX-4 which is described in U.S. Pat. No. 4,643,782, when it was tested for use in a gas turbine at a temperature of over 1000° C., underwent considerable coarsening of the γ′ phase, which disadvantageously leads to an increase in the creep rate of the alloy.
It is also necessary to improve the resistance of the known alloys to oxidation at very high temperatures. By way of example, U.S. Pat. No. 4,719,080 discloses that the addition of Pt, Pd, Ru and Os has a positive effect on an increase in the resistance of the single-crystal superalloys described in said document to oxidation and corrosion, where the overall content of these elements should lie in a very wide range of 0-10% by weight.
A further problem of the known nickel-base superalloys, for example the alloys which described in U.S. Pat. No. 5,435,861, is that in the case of large components, e.g. gas turbine blades or vanes with a length of more than 80 mm, the casting properties leave something to be desired. The casting of a perfect, relatively large directionally solidified single-crystal component from a nickel-base superalloy is extremely difficult, since most of these components have defects, e.g. small-angle grain boundaries, freckles (i.e. defects caused by a series of identically directed grains with a high eutectic content), equiaxed limits of variation, microporosities, etc. These defects weaken the components at high temperatures, and consequently the desired service life or operating temperature of the turbine are not achieved. However, since a perfectly cast single-crystal component is extremely expensive, the industry tends to permit as many defects as possible without the service life or operating temperature being adversely affected.
One of the most common defects is grain boundaries, which are particularly harmful to the high-temperature properties of the single-crystal components. Whereas in small components small-angle grain boundaries are relatively rare, they are highly relevant to the casting properties, the mechanical properties and the oxidation behavior of large SX or DS components at high temperatures.
Grain boundaries are regions with a high local disorder of the crystal lattice, since the neighboring grains collide in these regions, and therefore there is a certain misorientation between the crystal lattices. The greater the misorientation, the greater the disorder, i.e. the greater the number of dislocations in the grain boundaries which are required for the two grains to fit together. This disorder is directly related to the behavior of the material at high temperatures. It weakens the material if the temperature rises to above the equicohesive temperature (=0.5×melting point in K).
This effect is described in GB 2 234 521 A. For example, in a conventional nickel-base single-crystal alloy, at a test temperature of 871° C., the fracture strength drops greatly if the misorientation of the grains is greater than 6°. This has also been confirmed in single-crystal components with a directionally solidified microstructure, and consequently the viewpoint has generally been that misorientations of greater than 6° are unacceptable.
It is also described in the above-referenced GB 2 234 521 A that enriching nickel-base superalloys with boron or carbon during a directional solidification produces microstructures which have an equiaxed or prismatic grain structure. Carbon and boron strengthen the grain boundaries, since C and B cause the precipitation of carbides and borides at the grain boundaries, and these compounds are stable at high temperatures. Moreover, the presence of these elements in and along the grain boundaries reduces the diffusion process, which is a primary cause of the grain boundary weakness. It is therefore possible to increase the misorientations to 10° to 12° yet still achieve good materials properties at high temperatures. However, these small-angle grain boundaries have an adverse effect on the properties in particular of large single-crystal components formed from nickel-base superalloys.
EP 1 359 231 B1 discloses a nickel-base superalloy for the production of single-crystal components, which has improved casting properties and a higher resistance to oxidation than the abovementioned alloys and is characterized by the following chemical composition (details in % by weight):
7.7-8.3 Cr
5.0-5.25 Co
2.0-2.1 Mo
7.8-8.3 W
5.8-6.1 Ta
4.9-5.1 Al
1.3-1.4 Ti
0.11-0.15 Si
0.11-0.15 Hf
200-750 ppm C
50-400 ppm B
remainder nickel and production-related impurities.
After the casting process, superalloys of this type are subjected to heat treatment, in which, in a first solution annealing step, the γ′ phase which is precipitated nonuniformly during the casting process is completely or partially dissolved in the microstructure. In a second heat-treatment step, this phase is precipitated in a controlled manner again. In order to obtain optimal properties, this precipitation heat treatment is carried out in such a way that fine, uniformly distributed particles of the γ′ phase are produced in the γ phase (=matrix).
It has been found that targeted coarsening of the γ′ particles, the phenomenon known as rafting, occurs in the microstructure of alloys of this type if a mechanical load is present with long-term high-temperature loading (creep loading), or after plastic deformation of the material, which is followed by high-temperature loading of the material. At high γ′ contents (i.e. at a γ′ content of at least 50% by volume), the microstructure is inverted, i.e. γ′ becomes the continuous phase in which what was previously the γ matrix is embedded. A change in structure of this type is also brought about by plastic deformation of the superalloy, which is followed by heat treatment (high-temperature annealing).
Since the intermetallic γ′ phase tends toward environmental embrittlement, under certain loading conditions this subsequently leads to a massive drop in the mechanical properties, in particular the yield strength, at room temperature (25° C.) compared to samples which were not subjected to such prior creep loading. This impairment of the yield strength is described by the term “degradation” of the properties (see Pessah-Simonetti, P. Caron and T. Khan: Effect of long-term prior aging on tensile behaviour of high-performance single crystal superalloy, Journal de Physique IV, Colloque C7, Volume 3, November 1993).
However, if the tensile test is carried out at high test temperatures, for example 950° C., instead of at room temperature, this difference between the materials subjected to different loading with respect to the yield strength and the ductility does not arise or arises rarely.